Heme iron
Heme iron is an important source of dietary iron. It is a polyhedral compound, containing a group of iron ions, surrounded by porphyrin rings. In the body, heme is taken up by the intestinal lining and enterocytes and subsequently converted to non-iron. Iron-rich foods include fish, shellfish, meat, poultry, and beans.
Compared to inorganic iron, heme iron is absorbed more efficiently by the human intestine. The heme oxygenase is responsible for this. This enzyme is involved in releasing the iron from hemoglobin into the low molecular weight pool in enterocytes. While some inorganic iron forms become insoluble due to high pH of the upper small bowel, heme-iron is not affected. Interestingly, dietary fiber and tannins can inhibit the absorption of heme-iron.
Moreover, heme iron is able to bypass the usual feedback mechanism that governs iron absorption. Therefore, it is considered more body-friendly.
In addition, heme iron is not prone to the same health risks as non-heme iron. Nevertheless, a person can get enough iron from a plant-based diet without suffering from the side effects associated with excess iron intake. Nonetheless, it is highly recommended that people who do not consume animal-based food, or who do not follow a strict vegetarian diet, should get 1.8 times more iron than they normally do. If you want to increase your iron intake, try nuts, fortified cereals, legumes, and beans. Also, eat more fruits and vegetables. They contain a rich amount of vitamins and antioxidants, which have been shown to protect against chronic diseases.
Studies have shown that the amount of heme-iron absorbed is directly correlated to erythropoiesis rate and the risk of coronary heart disease (CHD). A study performed by Ascherio et al. examined the relationship between the intake of heme-iron and CHD in older adults. Similarly, a study conducted by Snowdon et al. investigated the association between the intake of heme and CHD in men. Lastly, a study by Tasevska et al. investigated the relationship between heme and lung cancer. These studies all showed that heme was associated with an increased risk of CHD. However, the results were not statistically significant.
Since heme is found in most animal-based foods, its uptake into the human body is very high. In fact, it has been discovered that heme is taken up by the intestinal mucosa and enterocytes in the same way as iron from dietary hemoglobin. Despite this, it has not been demonstrated that heme is reused by these cells.
Furthermore, heme-iron is taken up directly by cells that are not located in the gastrointestinal tract. For example, the Xenopus oocytes and HeLa cells have been shown to have a 2-to-3-fold increase in heme uptake.
There are also studies that indicate that the mRNA for HO-1 is constitutively expressed in the ileum, duodenum, and stomach, and that HCP1 is mRNA induced in hypoxia. Likewise, the HO-2 is constitutively expressed in brain and other tissues. Further research on the mRNA levels of these proteins is needed to determine if these proteins have any effect on the risk of CHD and other diseases.